This invention relates generally to temperature sensors of a type having application to heating ovens, and is particularly advantageously employed as a temperature sensor in domestic self-cleaning cooking ovens.
In self-cleaning domestic cooking ovens it is desirable to be able to control the oven temperature over an operating range on the order of 100.degree. F. to 1000.degree. F. or higher. In commercially available domestic thermal cooking ovens with electronic control systems, the temperature sensor typically comprises a sheathed length of nickel or nickel alloy wire which extends into the oven cavity along the rear interior wall. Temperature measurements are derived as a function of the resistance of the wire which changes with temperature. Such wire sensors are characterized by a relatively low temperature coefficient of resistivity. Thus, while the sensor itself is relatively inexpensive, the associated circuitry required to accurately detect relatively small changes in temperature for the range of operating temperatures over which the control system must operate is relatively costly.
Platinum resistance temperature detectors (RTD) of various types are also commercially available for measuring temperatures in the range of interest for self-cleaning oven applications. However, in addition to being relatively costly, RTD's have the same disadvantages as the nickel wire sensors, low temperature coefficient of resistivity over the temperature range of interest.
In commonly assigned U.S. Pat. No. 4,816,647 to Payne, a radiant cooktop appliance with a glass-ceramic cooktop surface includes a sensor for monitoring the temperature of the cooktop comprising a pair of parallel conductive strips deposited on the underside of the glass-ceramic surface. The surface resistance of the glass between the strips provides a measure of the temperature of the cooktop surface in that region. One disadvantage of such a sensor arrangement is its extremely high resistivity toward the lower end of the temperature range of interest for self-cleaning ovens, necessitating the use of additional circuitry to measure the temperature with sufficient accuracy over the entire operating range.
U.S. Pat. No. 3,786,390 to Kristen discloses a temperature sensor which senses temperature as a function of the bulk resistance of a glass-ceramic material between the conductive leads. However, it too is characterized by extremely high resistivity in the lower portion of the temperature range of interest.
Resistance thermometers employing a sensor structure comprising conductive film deposited on a glass substrate in a serpentine pattern to sense temperature as a function of the resistance of the conductive film itself is disclosed in U.S. Pat. No. 3,720,900. This sensor arrangement for use in precision scientific instruments to measure temperatures ranging from room temperature down to very low temperatures on the order of 10.degree. K. In this arrangement the glass merely serves to structurally support the conductive film. While such an arrangement might work well at the very low end of the cooking temperature range, it would likely not be satisfactory in the higher temperature and in any event such precision devices are too costly for domestic appliance applications.
Therefore, a need exists for a relatively inexpensive temperature sensor for electronically controlled domestic self-cleaning cooking ovens which provides better accuracy over the entire operating range than that achievable with conventional nickel wire sensors without need for the relatively expensive signal processing circuitry.
It is therefore an object of the present invention to provide a temperature sensor comprising relatively inexpensive materials, which is easily and inexpensively assembled and which operates over the range of 100.degree. F. to 1500.degree. F. with resistance values sufficiently low over this range to permit improved accuracy without need for complex and costly interface circuitry to process the temperature signals derived from the sensor.